Over the years, numerous methods have been proposed for improving the delivery of biologically active agents. Challenges associated with the formulation and delivery of pharmaceutical agents can include poor aqueous solubility of the pharmaceutical agent, toxicity, low bioavailability, instability, and rapid in-vivo degradation, to name just a few. Although many approaches have been devised for improving the delivery of pharmaceutical agents, no single approach is without its drawbacks. For instance, commonly employed drug delivery approaches aimed at solving or at least ameliorating one or more of these problems include drug encapsulation, such as in a liposome, polymer matrix, or unimolecular micelle, covalent attachment to a water-soluble polymer such as polyethylene glycol, use of gene targeting agents, and the like.
In looking more closely at some of these approaches, liposome encapsulation is often plagued by low efficiencies of drug loading, resulting in an oftentimes inefficient and cost ineffective process. Moreover, the release rate of the active agent in a liposomal formulation depends upon dissolution or disintegration of the liposome, or diffusion of the active agent through the liposomal layers, thereby limiting the practical availability of the active agent to the biological system. In addition, liposomal formulations are generally restricted to lipid soluble drugs. Polymer matrix-based formulations can suffer from similar shortcomings, such as the inability to well-characterize such drug delivery systems, particular those that are cross-linked, and the variable release rates associated with active agents that must diffuse out of a hydrolytically degradable polymer matrix. In comparison, conjugation of an active agent to a polymer such as polyethylene glycol offers a more well-defined alternative, since the conjugate itself is often although not necessarily well-characterized, particularly in the case of site-specific attachment of the polymer to the active agent. However, protein-based compositions containing mixtures of positional isomers varying in both the site(s) and number of polymer chains attached to a particular protein are not uncommon. This can lead to problems with reproducibly preparing such compositions.
While modification of therapeutic proteins for the purpose of improving their pharmaceutical utility is perhaps one of the most common applications of PEGylation, PEGylation has also been used, albeit to a limited degree, to improve the bioavailability and ease of formulation of small molecule therapeutics having poor aqueous solubilities. For instance, water-soluble polymers such as PEG have been covalently attached to artilinic acid to improve its aqueous solubility (Bentley, et al., U.S. Pat. No. 6,461,603). Similarly, PEG has been covalently attached to triazine-based compounds such as trimelamol to improve their solubility in water and enhance their chemical stability (Bentley, et al., WO 02/043772). Covalent attachment of PEG to bisindolyl maleimides has been employed to improve poor bioavailability of such compounds due to low aqueous solubility (Bentley, et al., WO 03/037384). Prodrugs of camptothecin having one or two molecules of camptothecin covalently attached to a linear polyethylene glycol have similarly been prepared (Greenwald, et al, U.S. Pat. No. 5,880,131).
Camptothecin (often abbreviated as “CPT”) is a phytotoxic alkaloid first isolated from the wood and bark of Camptotheca acuminata (Nyssaceae), and has been shown to exhibit antitumor activity. The compound has a pentacyclic ring system with an asymmetric center in lactone ring E with a 20 S configuration. The pentacyclic ring system includes a pyrrolo[3,4-b]quinoline (rings A, B and C), a conjugated pyridone (ring D), and a six-membered lactone (ring E) with a 20-hydroxyl group. Due to its insolubility in water, camptothecin was initially evaluated clinically in the form of a water-soluble carboxylate salt having the lactone ring open to form the sodium salt. The sodium salt, although exhibiting much improved water solubility in comparison to camptothecin itself, produced severe toxicity and demonstrated very little in vivo anticancer activity, thus demonstrating the undesirability of this approach.
It was later discovered that camptothecin and many of its derivatives inhibit topoisomerase, an enzyme that is required for swiveling and relaxation of DNA during molecular events such as replication and transcription. Camptothecin stabilizes and forms a reversible enzyme-camptothecin-DNA ternary complex. The formation of the cleavable complex specifically prevents the reunion step of the breakage/union cycle of the topoisomerase reaction. Topoisomerase I inhibitors are also known to be useful in the treatment of HIV.
In an effort to address the poor aqueous solubility associated with camptothecin and many of its derivatives, a number of synthetic efforts have been directed to derivatizing the A-ring and/or B-ring or esterifying the 20-hydroxyl to improve water-solubility while maintaining cytotoxic activity. For example, topotecan (9-dimethylaminomethyl-10-hydroxy CPT) and irinotecan (7-ethyl-10[4-(1-piperidino)-1-piperidino]carbonyloxy CPT), otherwise known as CPT-11, are two water-soluble CPT derivatives that have shown clinically useful activity. Conjugation of certain camptothecin derivatives, such as 10-hydroxycamptothecin and 11-hydroxycamptothecin, to a linear poly(ethylene glycol) molecule via an ester linkage has been described as a means to form water soluble prodrugs (Greenwald, et al., U.S. Pat. No. 6,011,042).
The clinical effectiveness of many small molecule therapeutics, and oncolytics in particular, is limited by several factors. For instance, irinotecan and other camptothecin derivatives undergo an undesirable hydrolysis of the E-ring lactone under alkaline conditions. Additionally, administration of irinotecan causes a number of troubling side effects, including leukopenia and diarrhea. Due to its severe diarrheal side-effect, the dose of irinotecan that can be administered in its conventional, unmodified form is extremely limited, thus hampering the efficacy of this drug and others of this type.
These associated side effects, when severe, can be sufficient to arrest further development of such drugs as promising therapeutics. Additional challenges facing small molecules include high clearance rates, and in the instance of anticancer agents, minimal tumor permeation and residence time. Approaches involving the use of polymer attachment must balance the size of the polymer against the molecular weight of the active agent in order to allow therapeutically effective doses to be delivered. Finally, the synthesis of a modified or drug-delivery enhanced active agent must result in reasonable yields, to make any such approach economically attractive. Thus, there exists a need for new methods for effectively delivering drugs, and in particular small molecule drugs, and even more particularly oncolytics, which can reduce their adverse and often toxic side-effects, whilst simultaneously improving their efficacy and ease of formulation. Specifically, there is a need for improved methods for delivering drugs that possess an optimal balance of bioavailability due to reduced clearance times, bioactivity, and efficacy, coupled with reduced side-effects. The present invention meets those needs.